A general radio base station (RBS) usually comprises different parts such as a radio equipment (RE) unit and a radio equipment controller unit (REC). The RE includes radio transceiver parts used to connect to one or several antennas and is responsible to perform e.g. conversion between digital baseband signals and analog radio frequency signals that interface with one or several antennas. The REC comprises all control functions ranging from the processing of radio information in the digital baseband domain to radio resource control layer functions. An interface named Common Public Radio Interface (CPRI) is used for handling communication between the REC and the RE parts. The CPRI interface is a standardized interface and is used for multiplexing In-Phase and Quadrature (IQ) data and synchronization and control/management of the RE onto a single link. The CPRI interface is currently specified for WCDMA (Wideband Code Division Multiple Access) and WIMAX (Worldwide Interoperability Microwave Access) systems. By using the CPRI interface, system/network designers have the flexibility to either co-locate the REC and the RE within a single enclosure or to remotely locate the RE from the REC in a distributed topology.
The main part of the CPRI interface bandwidth is used to convey baseband samples, so called IQ samples, of radio signals received and of radio signals to be transmitted. The IQ samples, which relates to user data, include phase and amplitude information. The CPRI interface also has the possibility to convey multiple different independent streams of IQ samples, where each bidirectional stream transported is the digital baseband data associated with the transmission and reception of one wireless carrier at one antenna. These IQ sample streams are referred to as antenna carriers (AxC) in the CPRI standard. Since one AxC represents the samples for one carrier on one antenna, then two carriers on each two antennas use four AxCs. The higher the sample rate of the AxC, the higher the bit rate usage on the CPRI interface. If, for example, a 20 MHz carrier is used on the radio interface, such as in LTE (Long Term Evolution) system, up to 30.72 Msamples/s per AxC are used. With 30 bit samples, a 2.4576 Gbps CPRI interface can convey two such 20 MHz LTE AxC.
The CPRI interface between the REC and the RE, is time divided into Basic Frames. A Basic Frame is the fundamental unit of the CPRI frame structure and is a 16-word frame. The first 1/16:th of the Basic Frame consists of a Control Word (CW) for synchronization and control signaling, and the following 15/16:th of the Basic Frame consists of the IQ field i.e. user data payload where the IQ samples are transmitted. Each Basic Frame is ( 1/3.84) micro seconds long (or approximately 260.42 ns long). Each AxC has an AxC Container defined in which it shall place its IQ samples. The AxC container points out what part of the IQ Field to use.
In the LTE system and other systems supporting multiple input multiple output (MIMO) antennas or beam forming, it gives a performance benefit to have complete control of the phase difference between the antennas, both in the uplink (UL) direction and in the downlink (DL) direction. To get control, calibration is needed. One good way is to measure the transmission, e.g. receive the own transmitted signal and feeding it back to the signal processing in the REC via the CPRI interface. Correspondingly, a well defined signal can be fed to the receivers and the distorted signal measured in the signal processing in the REC. When the REC receives calibration information it is able to compensate for phase distortion and for attenuation varieties by means of digital signal processing. These distortion and varieties are often caused by temperature variations in the analogue parts of the RE. The extra reception of signals, both for UL and DL calibration, therefore requires extra bandwidth on the CPRI interface.
It is previously known to have a separate calibration RE which can insert signals on the receive antennas, or receive signals from the transmit antennas that are connected to the RE. The signal to transmit and the signal to receive are, as previously described, defined as separate Antenna-Carriers (AxC) on the CPRI interface, thus allocating bandwidth.